U.S. patent application number 10/333535 was filed with the patent office on 2004-02-12 for field electron emission materials and devices.
Invention is credited to Burden, Adrian Paul, Edirisinghe, Mohan, Hood, Christopher, Lee, Warren, Tuck, Richard Allan, Waite, Michael Stuart.
Application Number | 20040025732 10/333535 |
Document ID | / |
Family ID | 9894648 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040025732 |
Kind Code |
A1 |
Tuck, Richard Allan ; et
al. |
February 12, 2004 |
Field electron emission materials and devices
Abstract
To create a field electron emission material, there is printed
upon a substrate (1501) an ink (1503) comprising a major component
of fluid vehicle; a first minor component of electrically
insulating material, either on its own or provided within a
precursor therefor; and a second minor component of electrically
conductive particles (1504). The printed ink is then treated to
expel the major component and create the field electron emission
material from the minor components on the substrate. The
electrically conductive particles may be omitted, to print a solid,
electrically insulating layer in a field emission device.
Inventors: |
Tuck, Richard Allan;
(Berkshire, GB) ; Burden, Adrian Paul;
(Oxfordshire, GB) ; Hood, Christopher; (Berkshire,
GB) ; Lee, Warren; (Berkshire, GB) ; Waite,
Michael Stuart; (Cheshire, GB) ; Edirisinghe,
Mohan; (London, GB) |
Correspondence
Address: |
Lee Mann Smith McWilliams Sweeney & Ohlson
P O Box 2786
Chicago
IL
60690-2786
US
|
Family ID: |
9894648 |
Appl. No.: |
10/333535 |
Filed: |
January 21, 2003 |
PCT Filed: |
June 28, 2001 |
PCT NO: |
PCT/GB01/02862 |
Current U.S.
Class: |
101/483 |
Current CPC
Class: |
H01J 9/025 20130101;
H01J 2201/30403 20130101 |
Class at
Publication: |
101/483 |
International
Class: |
B41C 001/00; B41M
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2000 |
GB |
0015928.5 |
Claims
1. A method of creating a field electron emission material,
comprising the steps of: a. printing upon a substrate an ink
comprising: i. a major component of fluid vehicle; ii. a first
minor component of electrically insulating material, either ready
formed or provided within a precursor therefor; and iii. a second
minor component of electrically conductive particles: and b.
treating the printed ink to expel said major component and create
said field electron emission material from said minor components on
said substrate.
2. A method according to claim 1, wherein said substrate has an
electrically conductive surface upon which said ink is printed.
3. A method according to claim 1 or 2, wherein said particles
comprise graphite.
4. A method according to claim 1, 2 or 3, wherein said particles
are predominantly acicular.
5. A method according to claim 1, 2 or 3, wherein said particles
are predominantly lamelliform.
6. A method according to claim 1, 2 or 3, wherein said particles
are predominantly equiaxed.
7. A method according to claim 1, 2 or 3, wherein said particles
have a low amorphous content.
8. A method according to claim 1 or 2, wherein said particles
comprise nanotubes of carbon or other materials.
9. A method according to claim 2 or to any of claims 3 to 8
together with claim 2, wherein said treatment of the printed ink is
such that each of said particles has a layer of said electrically
insulating material disposed in a first location between said
conductive surface and said particle, and/or in a second location
between said particle and the environment in which the field
electron emission material is disposed, such that electron emission
sites are formed at at least some of said first and/or second
locations.
10. A method according to any of the preceding claims, wherein said
particles are included within a mixture of a plurality of first
particles together with a plurality of second particles of
generally smaller dimensions than said first particles.
11. A method according to claim 10, wherein at least some of said
second particles decorate said first particles.
12. A method according to claim 10 or 11, wherein at least some of
said second particles are disposed in interstices defined between
said first particles.
13. A method according to claim 10, 11 or 12, wherein said second
particles comprise particles of at least two differing types.
14. A method according to any of claims 10 to 13, wherein some or
all of said second particles are more equiaxed than said first
particles.
15. A method according to any of claims 10 to 14, wherein some or
all of said second particles are more acicular than said first
particles.
16. A method according to any of claims 10 to 15, wherein said
first particles comprise graphite and said second particles
comprise carbon blacks.
17. A method according to any of claims 10 to 16, wherein said
first particles comprise graphite and said second particles
comprise fumed silica or Laponite.
18. A method according to any of claims 10 to 15, wherein said
first particles comprise a resistive material and said second
particles comprise graphite.
19. A method according to claim 18, wherein said first particles
comprise silicon carbide.
20. A method according to any of claims 10 to 19, wherein said
second particles have a higher BET surface area value than said
first particles.
21. A method according to any of claims 10 to 20, wherein said
second particles are more crystalline than said first
particles.
22. A method according to any of the preceding claims, wherein said
ink contains said precursor for said electrically insulating
material and said treatment of the printed ink includes subjecting
the printed ink to conditions in which said precursor is converted
into said electrically insulating material around at least part of
each of said conductive particles.
23. A method according to claim 22, wherein said conditions include
heating.
24. A method according to any of claims 1 to 21, wherein said
electrically insulating material is provided as a substantially
ready-formed layer on each of said electrically conductive
particles.
25. A method according to any of the preceding claims, including
the preliminary step of mixing said minor components and adding
them to said major component, thereby to form said ink.
26. A method of creating a solid, electrically insulating layer in
a field emission device, comprising the steps of: a. printing on a
substrate an ink comprising: i. a major component of fluid vehicle;
and ii. a minor component of electrically insulating material,
either ready formed or provided within a precursor therefor: and b.
treating the printed ink to expel said major component and create
said solid, electrically insulating layer from said minor component
on said substrate.
27. A method according to claim 26, wherein said solid,
electrically insulating layer is formed as a gate insulator.
28. A method according to any of the preceding claims, including
said precursor for said electrically insulating material, said
precursor being in the form of a sol-gel or polymer precursor.
29. A method according to claim 28, wherein said precursor is a
silica sol-gel.
30. A method according to claim 28, wherein said precursor is an
alumina sol-gel.
31. A method according to claim 28, wherein said precursor is a
polysiloxane.
32. A method according to claim 28, wherein said precursor is a
silsesquioxane polymer.
33. A method according to claim 32, wherein said silsesquioxane is
selected from the group comprising
.beta.-chloroethylsilsesquioxane; hydrogensilsequioxane; and
acetoxysilsesquioxane.
34. A method according to any of the preceding claims, wherein said
electrically insulating material is selected from the group
comprising amorphous silica; ormosils; amorphous alumina and
Laponite.
35. A method according to any of the preceding claims, wherein said
fluid vehicle comprises water.
36. A method according to any of the preceding claims, wherein said
fluid vehicle comprises an organic solvent.
37. A method according to any of the preceding claims, wherein said
fluid vehicle contains at least one additive to control the
rheology of the ink.
38. A method according to claim 37, wherein said at least one
additive includes at least one thickening agent.
39. A method according to claim 38, wherein said thickening agent
comprises a fugitive soluble organic polymer.
40. A method according to claim 39, wherein said fugitive soluble
organic polymer is selected from the group comprising poly(vinyl
alcohol; ethyl cellulose; hydroxyethyl cellulose; carboxymethyl
cellulose; methylhydroxypropyl cellulose; hydroxypropyl cellulose;
xanthan gum; and guar gum.
41. A method according to claim 38, wherein said thickening agent
comprises a non-fugitive material.
42. A method according to claim 41 and to any of claims 1 to 25,
wherein said non-fugitive material is selected from the group
comprising fumed silica; carbon blacks; and Laponite.
43. A method according to any of claims 37 to 42, comprising at
least one further additive to control further properties of the
ink.
44. A method according to claim 43, wherein said at least one
further additive comprises at least one of an anti-foaming agent; a
levelling agent; a vetting agent; a preservative; an air-release
agent; a retarder, and a dispersing agent.
45. A method according to claim 44, wherein said anti-foaming agent
is a fugitive material.
46. A method according to claim 45, wherein said fugitive material
is selected from the group comprising butyl cellosolve; n-octanol;
emulsions of organic polymers and organic metal-compounds; and
silicone-free defoaming substances in alkylbenezene.
47. A method according to claim 44, wherein said anti-foaming agent
is a non fugitive material.
48. A method according to claim 47, wherein said non-fugitive
material comprises a silicone.
49. A method according to any of claims 44 to 48, wherein said
dispersing agent is selected from the group comprising poly(vinyl
alcohol; modified polyurethane in butylacetate,
methoxypropylacetate and sec. butanol; modified polyacrylate in
meythoxypropanol; polyethylene glycol
mono(4-(1,1,3,3-tetramethylbutyl)phenyl)ether; and mineral
oils.
50. A method according to claim 49, wherein said dispersing agent
comprises a silicone oil.
51. A method according to any of claims 44 to 50, wherein said at
least one further additive comprises at least one dispersing agent
and at least one said minor component has an affinity for that
dispersing agent.
52. A method according to any of claims 44 to 51, wherein said
levelling agent is selected from the group comprising poly(vinyl)
alcohol; fluorocarbon modified polyacrylate in sec. butanol;
organically modified polysiloxane in isobutanol; and solvent-free
modified polysiloxane.
53. A method according to any of claims 44 to 52, wherein said
wetting agent is selected from the group comprising unsaturated
polyamide and acid ester salt in xylene, n-butanol and
monpropylenegylcol; and alkylol ammonium salt of a high molecular
weight carboxylic acid in water.
54. A method according to any of claims 44 to 53, wherein said
preservative is selected from the group comprising phenols and
formaldehydes.
55. A method according to any of claims 44 to 54, wherein said
air-release agent is selected from the group comprising silica
particles and silicones.
56. A method according to any of claims 44 to 55, wherein said
retarder is selected from the group comprising 1,2-propanediol and
terpineol.
57. A method according to any of the preceding claims, wherein said
printing comprises screen printing.
58. A method according to any of the preceding claims, wherein said
printing comprises ink-jet printing.
59. A method according to any of claims 1 to 56, wherein said
printing is selected from the group comprising offset lithography;
pad printing; table coating and slot printing.
60. A method according to any of the preceding claims, wherein said
substrate is porous and said step of treating the printed ink
includes absorbing at least part of said fluid vehicle into said
porous substrate.
61. A method according to any of the preceding claims, wherein said
step of treating the printed ink causes the mean thickness of the
insulator in the cured layer to be reduced to 10% or less of the
thickness of the ink as printed.
62. A method according to any of the preceding claims, wherein said
step of treating the printed ink causes the mean thickness of the
insulator in the cured layer to be reduced to 5% or less of the
thickness of the ink as printed.
63. A method according to any of the preceding claims, wherein said
step of treating the printed ink causes the mean thickness of the
insulator in the cured layer to be reduced to 1% or less of the
thickness of the ink as printed.
64. A method according to any of the preceding claims, wherein said
step of treating the printed ink causes the mean thickness of the
insulator in the cured layer to be reduced to 0.5% or less of the
thickness of the ink as printed.
65. A method according to any of the preceding claims, wherein said
major component comprises at least 50% by weight of the ink.
66. A method according to any of the preceding claims, wherein said
major component comprises at least 80% by weight of the ink.
67. A method according to any of the preceding claims, wherein said
major component comprises at least 90% by weight of the ink.
68. A method according to any of the preceding claims, wherein said
major component comprises at least 95% by weight of the ink.
69. A method according to any of the preceding claims, wherein the
weight of the or each said minor component in total comprises less
than 50% by weight of the ink.
70. A method according to any of the preceding claims, wherein the
weight of the or each said minor component in total comprises less
than 10% by weight of the ink.
71. A method according to any of the preceding claims, wherein the
weight of the or each said minor component in total comprises less
than 5% by weight of the ink.
72. A method according to any of the preceding claims, wherein the
weight of the or each said minor component in total comprises less
than 2% by weight of the ink.
73. A method according to any of the preceding claims, wherein the
weight of the or each said minor component in total comprises less
than 1% by weight of the ink.
74. An method of creating a field electron emission material,
substantially as hereinbefore described with reference to the
accompanying drawings.
75. A field electron emitter comprising field electron emission
material that has been created by a method according to any of the
preceding claims.
76. A field electron emission device comprising a field electron
emitter according to claim 75, and means for subjecting said
emitter to an electric field in order to cause said emitter to emit
electrons.
77. Afield electron emission device according to claim 76,
comprising a substrate with an array of patches of said field
electron emitters, and control electrodes with aligned arrays of
apertures, which electrodes are supported above the emitter patches
by insulating layers.
78. A field electron emission device according to claim 77, wherein
said apertures are in the form of slots.
79. Afield electron emission device according to any of claims 76
to 78, comprising a plasma reactor, corona discharge device, silent
discharge device, ozoniser, an electron source, electron gun,
electron device, x-ray tube, vacuum gauge, gas filled device or ion
thruster.
80. A field electron emission device according to any of claims 76
to 79, wherein the field electron emitter supplies the total
current for operation of the device.
81. A field electron emission device according to any of claims 76
to 80, wherein the field electron emitter supplies a starting,
triggering or priming current for the device.
82. A field electron emission device according to any of claims 76
to 81, comprising a display device.
83. Afield electron emission device according to any of claims 76
to 81, comprising a lamp.
84. Afield electron emission device according to claim 83, wherein
said lamp is substantially flat.
85. A field electron emission device according to any of claims 76
to 84, wherein said emitter is connected to an electric driving
means via a ballast resistor to limit current.
86. A field electron emission device according to claims 77 and 85,
wherein said ballast resistor is applied as a resistive pad under
each said emitting patch.
87. A field electron emission device according to any of claims 76
to 86, wherein said emitter material and/or a phosphor is/are
coated upon one or more one-dimensional array of conductive tracks
which are arranged to be addressed by electronic driving means so
as to produce a scanning illuminated line.
88. A field electron emission device according to claim 87,
including said electronic driving means.
89. A field electron emission device according to any of claims 76
to 88, wherein said field emitter is disposed in an environment
which is gaseous, liquid, solid, or a vacuum.
90. A field electron emission device according to any of claims 76
to 89, comprising a cathode which is optically translucent and is
so arranged in relation to an anode that electrons emitted from the
cathode impinge upon the anode to cause electro-luminescence at the
anode, which electro-luminescence is visible through the optically
translucent cathode.
91. A field electron emission device, substantially as hereinbefore
described with reference to the accompanying drawings.
Description
[0001] This invention relates to field electron emission materials,
and devices using such materials.
[0002] In classical field electron emission, a high electric field
of, for example, .apprxeq.3.times.10.sup.9 V m.sup.-1 at the
surface of a material reduces the thickness of the surface
potential barrier to a point at which electrons can leave the
material by quantum mechanical tunnelling. The necessary conditions
can be realised using atomically sharp points to concentrate the
macroscopic electric field. The field electron emission current can
be further increased by using a surface with a low work function.
The metrics of field electron emission are described by the
well-known Fowler-Nordheim equation.
[0003] There is considerable prior art relating to tip based
emitters, which term describes electron emitters and emitting
arrays which utilise field electron emission from sharp points
(tips). The main objective of workers in the art has been to place
an electrode with an aperture (the gate) less than 1 .mu.m away
from each single emitting tip, so that the required high fields can
by achieved using applied potentials of 100V or less--these
emitters are termed gated arrays. The first practical realisation
of this was described by C A Spindt, working at Stanford Research
Institute in California (J.Appl.Phys. 39,7, pp 3504-3505, (1968)).
Spindt's arrays used molybdenum emitting tips which were produced,
using a self masking technique, by vacuum evaporation of metal into
cylindrical depressions in a SiO.sub.2 layer on a Si substrate.
[0004] In the 1970s, an alternative approach to produce similar
structures was the use of directionally solidified eutectic alloys
(DSE). DSE alloys have one phase in the form of aligned fibres in a
matrix of another phase. The matrix can be etched back leaving the
fibres protruding. After etching, a gate structure is produced by
sequential vacuum evaporation of insulating and conducting layers.
The build up of evaporated material on the tips acts as a mask,
leaving an annular gap around a protruding fibre.
[0005] An important approach is the creation of gated arrays using
silicon micro-engineering. Field electron emission displays
utilising this technology are being manufactured at the present
time, with interest by many organisations world-wide.
[0006] Major problems with all tip-based emitting systems are their
vulnerability to damage by ion bombardment, ohmic heating at high
currents and the catastrophic damage produced by electrical
breakdown in the device. Making large area devices is both
difficult and costly.
[0007] In about 1985, it was discovered that thin films of diamond
could be grown on heated substrates from a hydrogen-methane
atmosphere, to provide broad area field emitters--that is, field
emitters that do not require deliberately engineered tips.
[0008] In 1991, it was reported by Wang et al (Eletron. Lett., 27,
pp 1459-1461 (1991)) that field electron emission current could be
obtained from broad area diamond films with electric fields as low
as 3 MV m.sup.-1. This performance is believed by some workers to
be due to a combination of the low electron affinity of the (111)
facets of diamond and the high density of localised, accidental
graphite inclusions (Xu, Latham and Tzeng. Electron. Lett., 29, pp
1596-159(1993)) although other explanations are proposed.
[0009] Coatings with a high diamond content can now be grown on
room temperature substrates using laser ablation and ion beam
techniques. However, all such processes utilise expensive capital
equipment and the performance of the materials so produced is
unpredictable.
[0010] SI Diamond in the USA has described a field electron
emission display (FED) that uses as the electron source a material
that it calls Amorphic Diamond. The diamond coating technology is
licensed from the University of Texas. The material is produced by
laser ablation of graphite onto a substrate.
[0011] From the 1960s onwards another group of workers has been
studying the mechanisms associated with electrical breakdown
between electrodes in vacuum. It is well known (Latham and Xu,
Vacuum, 42, 18, pp 1173-1181 (1991)) that as the voltage between
electrodes is increased no current flows until a critical value is
reached at which time a small noisy current starts flowing. This
current increases both monotonically and stepwise with electric
field until another critical value is reached, at which point it
triggers an arc. It is generally understood that the key to
improving voltage hold-off is the elimination of the sources of
these pre-breakdown currents. Current understanding shows that the
active sites are metal-insulator-vacuum (MIV) structures formed by
either embedded dielectric particles or conducting flakes sitting
on insulating patches such as the surface oxide of the metal. In
both cases, the current comes from a hot electron process that
accelerates the electrons resulting in quasi-thermionic emission
over the surface potential barrier. This is well described in the
scientific literature e.g. Latham, High Voltage Vacuum Insulation,
Academic Press (1995). Although the teachings of this work have
been adopted by a number of technologies (e.g. particle
accelerators) to improve vacuum insulation, until recently little
work has been done to create field electron emitters using the
knowledge.
[0012] Latham and Mousa (J. Phys.D: Appl. Phys. 19, pp 699-713
(1986)) describe composite metal-insulator tip-based emitters using
the above hot electron process and in 1988 S Bajic and R V Latham
(Journal of Physics D Applied Physics, vol. 21 200-204 (1988)),
described a composite that created a high density of
metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. The
composite had conducting particles dispersed in an epoxy resin. The
coating was applied to the surface by standard spin coating
techniques.
[0013] Much later in 1995 Tuck, Taylor and Latham (GB 2 304 989)
improved the above MIMIV emitter by replacing the epoxy resin with
an inorganic insulator that both improved stability and enabled it
to be operated in sealed off vacuum devices. In 1997 Tuck and
Bishop (GB 2 332 089) described electron emitters using
metal-insulator-vacuum (MIV) emitter sites.
[0014] Embodiments of the present invention aim to provide inks for
use in creating broad area field emitting materials, that maybe
printed by means of silk screen, offset lithography and other
techniques.
[0015] Preferred embodiments of the present invention aim to
provide cost-effective broad area field emitting materials and
devices that maybe used in devices that include (amongst others):
field electron emission display panels; high power pulse devices
such as electron MASERS and gyrotrons; crossed-field microwave
tubes such as CFAs; linear beam tubes such as klystrons; flash
x-ray tubes; triggered spark gaps and related devices; broad area
x-ray sources for sterilisation; vacuum gauges; ion thrusters for
space vehicles; particle accelerators; ozonisers; and plasma
reactors.
[0016] According to one aspect of the present invention, there is
provided a method of creating a field electron emission material,
comprising the steps of:
[0017] a. printing upon a substrate an ink comprising:
[0018] i. a major component of fluid vehicle;
[0019] ii. a first minor component of electrically insulating
material, either ready formed or provided within a precursor
therefor; and
[0020] iii. a second minor component of electrically conductive
particles: and
[0021] b. treating the printed ink to expel said major component
and create said field electron emission material from said minor
components on said substrate.
[0022] In the context of this specification, printing means a
process that places an ink in a defined pattern. Examples of
suitable processes are (amongst others): screen printing,
Xerography, photolithography, electrostatic deposition, spraying,
ink jet printing and offset lithography.
[0023] As will be understood by those skilled in the art, in the
context of this specification, references to printing an ink upon a
substrate include printing both directly on the substrate and also
upon a layer or component that already exists upon the
substrate.
[0024] Preferably, said substrate has an electrically conductive
surface upon which said ink is printed.
[0025] Preferably, said particles comprise graphite.
[0026] Said particles maybe predominantly acicular.
[0027] Said particles may be predominantly lamelliform.
[0028] Said particles maybe predominantly equiaxed.
[0029] Preferably, said particles have a low amorphous content.
[0030] By particles of low amorphous content we mean materials
where the amorphous content is less than 5% and, preferably, where
the amorphous content cannot be detected by x-ray diffraction
analysis. This means that the amorphous component is less than 1%
or, in many cases, less than 0.1%. Byway of example, such particles
maybe prepared from well crystallised feedstocks by jet-milling.
This may apply especially to graphite particles.
[0031] Said particles may comprise nanotubes of carbon or other
materials.
[0032] Preferably, said treatment of the printed ink is such that
each of said particles has a layer of said electrically insulating
material disposed in a first location between said conductive
surface and said particle, and/or in a second location between said
particle and the environment in which the field electron emission
material is disposed, such that electron emission sites are formed
at at least some of said first and/or second locations.
[0033] Said particles may be included within a mixture of a
plurality of first particles together with a plurality of second
particles of generally smaller dimensions than said first
particles.
[0034] At least some of said second particles may decorate said
first particles.
[0035] At least some of said second particles maybe disposed in
interstices defined between said first particles.
[0036] Said second particles may comprise particles of at least two
differing types.
[0037] Said second particles maybe more equiaxed than said first
particles.
[0038] Said second particles maybe more acicular than said first
particles.
[0039] Said first particles may comprise graphite and said second
particles may comprise carbon blacks.
[0040] Said first particles may comprise graphite and said second
particles may comprise fumed silica or Laponite.
[0041] Said first particles may comprise a resistive material and
said second particles may comprise graphite.
[0042] Said first particles may comprise silicon carbide.
[0043] Said second particles may have a higher BET surface area
value than said first particles.
[0044] Said second particles may be more crystalline than said
first particles.
[0045] Said ink may contain said precursor for said electrically
insulating material and said treatment of the printed ink may
include subjecting the printed ink to conditions in which said
precursor is converted into said electrically insulating material
around at least part of each of said conductive particles.
[0046] Said conditions may include heating.
[0047] Said electrically insulating material may be provided as a
substantially ready-formed layer on each of said electrically
conductive particles.
[0048] Any method as above may include the preliminary step of
mixing said minor components and adding them to said major
component, thereby to form said ink.
[0049] In another aspect, the present invention provides a method
of creating a solid, electrically insulating layer in a field
emission device, comprising the steps of:
[0050] a. printing on a substrate an ink comprising:
[0051] i. a major component of fluid vehicle; and
[0052] ii. a minor component of electrically insulating material,
either ready formed or provided within a precursor therefor:
and
[0053] b. treating the printed ink to expel said major component
and create said solid, electrically insulating layer from said
minor component on said substrate.
[0054] Said solid, electrically insulating layer maybe formed as a
gate insulator.
[0055] Any method as above may include said precursor for said
electrically insulating material, said precursor being in the form
of a sol-gel or polymer precursor.
[0056] Said precursor maybe a silica sol-gel.
[0057] Said precursor may be an alumina sol-gel.
[0058] Said precursor may be a polysiloxane.
[0059] Said precursor maybe a silsesquioxane polymer.
[0060] Preferably, said silsesquioxane is selected from the group
comprising .beta.-chloroethylsilsesquioxane; hydrogensilsequioxane;
and acetoxysilsesquioxane.
[0061] Said electrically insulating material may be selected from
the group comprising amorphous silica; ormosils; amorphous alumina
and Laponite.
[0062] Said fluid vehicle may comprise water.
[0063] Said fluid vehicle may comprise an organic solvent.
[0064] Said fluid vehicle may contain at least one additive to
control the rheology of the ink.
[0065] Preferably, said at least one additive includes at least one
thickening agent.
[0066] Said thickening agent may comprise a fugitive soluble
organic polymer.
[0067] In the context of this specification, the term "fugitive"
means a material expected to be consumed (for example to "burn
out") completely during treatment (for example, curing or firing),
and those skilled in the art will recognise that a small quantity
of non-detrimental ash or residue may nevertheless remain in some
instances.
[0068] Preferably, said fugitive soluble organic polymer is
selected from the group comprising poly(vinyl alcohol; ethyl
cellulose; hydroxyethyl cellulose; carboxymethyl cellulose;
methylhydroxypropyl cellulose; hydroxypropyl cellulose; xanthan
gum; and guar gum.
[0069] Said thickening agent may comprise a non-fugitive
material.
[0070] Preferably, said non-fugitive material is selected from the
group comprising fumed silica; carbon blacks; and Laponite.
[0071] A method as above may comprise at least one further additive
to control further properties of the ink.
[0072] Preferably, said at least one further additive comprises at
least one of an anti-foaming agent; a levelling agent; a wetting
agent; a preservative; an air-release agent; a retarder; and a
dispersing agent.
[0073] Any such further additive may perform more than one such
function.
[0074] Said anti-foaming agent maybe a fugitive material.
[0075] Preferably, said fugitive material is selected from the
group comprising butyl cellosolve; n-octanol; emulsions of organic
polymers and organic metal-compounds; and silicone-free defoaming
substances in alkylbenezene.
[0076] Said anti-foaming agent maybe a non-fugitive material.
[0077] Preferably, said non-fugitive material comprises a
silicone.
[0078] Preferably, said dispersing agent is selected from the group
comprising poly(vinyl) alcohol; modified polyurethane in
butylacetate, methoxypropylacetate and sec. butanol; modified
polyacrylate in meythoxypropanol; polyethylene glycol
mono(4(1,1,3,3-tetramethylbutyl)phe- nyl)ether; and mineral
oils.
[0079] Preferably, said said dispersing agent comprises a silicone
oil.
[0080] Said at least one further additive may comprise at least one
dispersing agent and at least one said minor component may have an
affinity for that dispersing agent.
[0081] Preferably, said said levelling agent is selected from the
group comprising poly(vinyl) alcohol; fluorocarbon modified
polyacrylate in sec. butanol; organically modified polysiloxane in
isobutanol; and solvent-free modified polysiloxane.
[0082] Preferably, said wetting agent is selected from the group
comprising unsaturated polyamide and acid ester salt in xylene,
n-butanol and monpropylenegylcol; and alkylol ammonium salt of a
high molecular weight carboxylic acid in water.
[0083] Preferably, said preservative is selected from the group
comprising phenols and formaldehydes.
[0084] Preferably, said air-release agent is selected from the
group comprising silica particles and silicones.
[0085] Preferably, said retarder is selected from the group
comprising 1,2-propanediol and terpineol.
[0086] Said printing may comprise screen printing.
[0087] Said printing may comprise ink-jet printing.
[0088] Said printing maybe selected from the group comprising
offset lithography; pad printing; table coating and slot
printing.
[0089] Preferably, said said substrate is porous and said step of
treating the printed ink includes absorbing at least part of said
fluid vehicle into said porous substrate.
[0090] Preferably, said said step of treating the printed ink
causes the mean thickness of the insulator in the cured layer to be
reduced to 10% or less of the thickness of the ink as printed.
[0091] The mean thickness of the insulator is the average height of
the insulator above the substrate on which it is disposed, away
from any other solid components of the ink such as said
electrically conductive particles. In the vicinity of such
particles, the thickness of the insulator can be influenced by the
surface area and morphology of the particles. By "away from" said
components such as said particles, we mean a distance of at least a
particle's mean radius from its perimeter.
[0092] Preferably, said step of treating the printed ink causes the
mean thickness of the insulator in the cured layer to be reduced to
5% or less of the thickness of the ink as printed.
[0093] Preferably, said step of treating the printed ink causes the
mean thickness of the insulator in the cured layer to be reduced to
1% or less of the thickness of the ink as printed.
[0094] Preferably, said said step of treating the printed ink
causes the mean thickness of the insulator in the cured layer to be
reduced to 0.5% or less of the thickness of the ink as printed.
[0095] Preferably, said major component comprises at least 50% by
weight of the ink.
[0096] Preferably, said major component comprises at least 80% by
weight of the ink.
[0097] Preferably, said major component comprises at least 90% by
weight of the ink.
[0098] Preferably, said major component comprises at least 95% by
weight of the ink.
[0099] Preferably, the weight of the or each said minor component
in total comprises less than 50% by weight of the ink.
[0100] Preferably, the weight of the or each said minor component
in total comprises less than 10% by weight of the ink.
[0101] Preferably, the weight of the or each said minor component
in total comprises less than 5% by weight of the ink.
[0102] Preferably, the weight of the or each said minor component
in total comprises less than 2% by weight of the ink.
[0103] Preferably, the weight of the or each said minor component
in total comprises less than 1% by weight of the ink.
[0104] The invention extends to a field electron emitter comprising
field electron emission material that has been created by a method
according to any of the preceding aspects of the invention.
[0105] The invention also extends to a field electron emission
device comprising a field electron emitter as above and means for
subjecting said emitter to an electric field in order to cause said
emitter to emit electrons.
[0106] Such a field electron emission device may comprise a
substrate with an array of patches of said field electron emitters,
and control electrodes with aligned arrays of apertures, which
electrodes are supported above the emitter patches by insulating
layers.
[0107] Said apertures maybe in the form of slots.
[0108] A field electron emission device as above may comprise a
plasma reactor, corona discharge device, silent discharge device,
ozoniser, an electron source, electron gun, electron device, x-ray
tube, vacuum gauge, gas filled device or ion thruster.
[0109] In a field electron emission device as above, the field
electron emitter may supply the total current for operation of the
device.
[0110] In a field electron emission device as above, the field
electron emitter may supply a starting, triggering or priming
current for the device.
[0111] A field electron emission device as above may comprise a
display device.
[0112] Afield electron emission device as above may comprise a
lamp.
[0113] Said lamp may be substantially flat.
[0114] Said emitter maybe connected to an electric driving means
via a ballast resistor to limit current.
[0115] Said ballast resistor may be applied as a resistive pad
under each said emitting patch.
[0116] Said emitter material and/or a phosphor may be coated upon
one or more one-dimensional array of conductive tracks which are
arranged to be addressed by electronic driving means so as to
produce a scanning illuminated line.
[0117] Such a field electron emission device may include said
electronic driving means.
[0118] Said field emitter maybe disposed in an environment which is
gaseous, liquid, solid, or a vacuum.
[0119] A field electron emission device as above may comprise a
cathode which is optically translucent and is so arranged in
relation to an anode that electrons emitted from the cathode
impinge upon the anode to cause electro-luminescence at the anode,
which electro-luminescence is visible through the optically
translucent cathode.
[0120] It will be appreciated that the electrical terms
"conducting" and "insulating" can be relative, depending upon the
basis of their measurement. Semiconductors have useful conducting
properties and, indeed, maybe used in the present invention as
conducting particles. In the context of this specification, each
said conductive particle has an electrical conductivity at least
10.sup.2 times (and preferably at least 10.sup.3 or 10.sup.4 times)
that of the insulating material.
[0121] For a better understanding of the invention, and to show how
embodiments of the same may be carried into effect, reference will
now be made, byway of example, to the accompanying diagrammatic
drawings, in which:
[0122] FIG. 1 shows a MIMIV field emitter material;
[0123] FIGS. 2a and 2b shows the dimensions of field emitting
layers as deposited by spin coating and after subsequent
firing;
[0124] FIG. 3 shows the dimensions of a field emitting layer as
deposited by screen printing;
[0125] FIG. 4 shows the natural orientation of a low concentration
of particles in a printed emitter layer;
[0126] FIG. 5 shows the orientation of a higher concentration of
particles in a printed emitter layer;
[0127] FIG. 6 shows how a gap-filling phase maybe used to increase
the density of a printed emitter layer;
[0128] FIG. 7a shows how separate layers within a thick emitter
function as emitter and resistive ballast;
[0129] FIG. 7b shows how insulating particles can be introduced
into a thick emitter layer;
[0130] FIGS. 8a to 8c show respective examples of field-emitting
devices using materials as disclosed herein;
[0131] FIG. 9a shows an emission image of a cathode;
[0132] FIG. 9b shows a voltage-current characteristics for a
cathode;
[0133] FIGS. 10a and 10b show two frequency histograms of threshold
fields using two different sized probes;
[0134] FIG. 11 shows an emission image of another cathode;
[0135] FIG. 12 shows emission characteristics measure using
scanning probe anodes;
[0136] FIG. 13 shows rheometric data for a typical ink described
herein;
[0137] FIG. 14 shows examples of fine feature printing using inks
described herein; and
[0138] FIGS. 15a and 15b illustrate how printing and emitting
properties maybe adjusted by controlling the porosity of a
substrate.
[0139] In the figures, like reference numerals denote like or
corresponding parts.
[0140] The invention may have many different embodiments, and
several examples are given in the following description. It is to
be appreciated that, where practical, features of one embodiment or
example can be used with features of other embodiments or
examples.
[0141] FIG. 1 shows a MIMIV emitter material as described by Tuck,
Taylor and Latham (GB 2 304 989) with electrically conducting
particles 11 in an inorganic, electrically insulating matrix 12 on
an electrically conducting substrate 13. For insulating substrates
13, a conducting layer 14 is applied before coating. The conducting
layer 14 may be applied by a variety of means including, but not
limited to, vacuum and plasma coating, electroplating, electroless
plating and ink based methods.
[0142] The emission process is believed to occur as follows.
Initially the insulator 12 forms a blocking contact between the
particles 11 and the substrate 13. The voltage of a particle will
rise to the potential of the highest equipotential it probes--this
has been called the antenna effect. At a certain applied voltage,
this will be high enough to create an electro-formed conducting
channel 17 between the particle and the substrate. The potential of
the particle then flips rapidly towards that of the substrate 13 or
conducting layer 14, typically arranged as a cathode track. The
residual charge above the particle then produces a high electric
field which creates a second electro-formed channel 18 and an
associated metal-insulator-vacuum (MIV) hot electron emission site.
After this switch-on process, reversible field emitted currents 20
can be drawn from the site.
[0143] The standing electric field required to switch on the
electro-formed channels is determined by the ratio of particle
height 16 to the thickness of the matrix in the region of the
conducting channels 15. For a minimum switch on field, the
thickness of the matrix 12 at the conducting channels should be
significantly less than the particle height. The conducting
particles would typically be in, although not restricted to, the
range 0.1 micrometres to 400 micrometres, preferably with a narrow
size distribution.
[0144] By a "channel", "conducting channel" or electro-formed
channel" we mean a region of the insulator where its properties
have been locally modified, usually by some forming process
involving charge injection or heat. Such a modification facilitates
the injection of electrons from the conducting back contact into
the insulator such that the electrons may move through it, gaining
energy, and be emitted over or through the surface potential
barrier into the vacuum. In a crystalline solid the injection maybe
directly into the conduction band or, in the case of amorphous
materials, at an energy level where hopping conduction is
possible.
[0145] Depositing a MIMIV or MIV emitter by printing, particularly
screen printing, presents a challenge. In the past, the Applicants
have deposited emitters by spin coating followed by firing. FIG. 2b
shows the idealised structure of a heat-treated layer with
conducting substrate 21, insulator layer 22 and conducting
particles 25. We have found that the optimum mean insulator
thickness 24 is approximately 100 nm: however, the insulator
thickness over the tops of the particles 25 should be approximately
20 nm. FIG. 2a shows the as-spun layer before heat treatment, where
the overall thickness of insulator precursor material 26 is of the
same order of thickness as the heat-treated layers. Spin coating
inks have a low viscosity and, as a result, suspensions of
particles in them must be frequently agitated. Liquids of this
viscosity, even with the aid of dispersing agents, cannot prevent
particle clumping once a critical concentration is exceeded. This
concentration is well below the ideal level for optimised
emitters.
[0146] So far as printing field emitting structures based upon
particulates is concerned, the distinct trend in the art has been
to emulate normal thick film circuit practice and use an ink in the
form of a paste. See, for example, Tcherepanov et al Pro.
Tri-service/NASA Cathode Workshop, Cleveland, Ohio (1994), EP 0 905
737 A1; KR 99-18948; KR 99-12717; KR99-15280. By a "paste" we mean
a malleable mixture wherein the particulate components comprise the
majority of the formulation and wherein the rheological, and hence
printing, properties are controlled to a large degree by friction
between said particulate components.
[0147] An alternative that has been tried (KR 2000-20870) is to
form a slurry of particles and insulator precursor that is viscous
enough to enable a higher concentration of particles but still
sufficiently liquid to be spin coated into a layer, albeit not a
classic single layer MIMIV or MIV structure as described above.
Such a slurry provides the worst of both worlds, for it is too
viscous for inkjet printing and too liquid to screen print. The
authors patterned their emitter films by means of a
photolithographic lift-off process.
[0148] Preferred embodiments of the present invention provide
methods of screen printing inks for classic MIMIV and MIV
structures, which meet a challenge which is illustrated in FIG. 3.
The ink's viscosity can now be much higher than previously
proposed, and so particle clumping is much less of an issue, but
the as-deposited layer thickness 31 is now approximately 20
micrometre. On heat-treatment we require this to shrink
controllably to produce good quality films of the known optimum
dimensions as shown in FIG. 2b. We shall call these "controllable
high thickness reduction inks" (CHTR inks).
[0149] MIMIV and MIV emitter layer coatings contain two essential
components:
[0150] 1. Conductive particles; and
[0151] 2. An insulator phase.
[0152] In the case of CHTR screen printing inks used to apply MIMIV
and MIV emitter coatings, they may also contain other components
(often temporary), added to control the rheological or other
properties required during the application process. Fillers such as
clays or fumed silica may be added to control the rheology of the
ink.
[0153] Laponite, for example, is a synthetic clay with flakes of 25
nm mean diameter and has a profound effect on the viscosity of
aqueous solutions by forming sol-gel solutions. Latexes may also be
used to control viscosity. Many organic polymers, which can also be
used, give a residue on thermal decomposition (often called "burn
out" in the art). The residue may typically comprise carbon and/or
salts and/or silica. Such additional materials may be removed after
they have served their purpose during application and curing
stages. Post-application treatment (usually heating) may also be
necessary to convert precursor materials into final forms required
for functional components of an insulator coating.
[0154] Emitter particles are most conveniently added to ink ready
formed from the desired material and with the desired particle size
distribution. However, treatments such as thermal decomposition,
chemical reduction or other reactions may be used to transform a
precursor material into the form required in the emitting
material.
[0155] An insulator phase is preferably present as a thin,
continuous layer over the whole emitter surface and, in its final
form, it must be stable indefinitely under high vacuum. Although it
is easy to form an insulating layer from organic polymers (e.g. S
Bajic and R V Latham, (Journal of Physics D Applied Physics, vol 21
200-204 (1988)) and these have been shown to operate in a
continuously pumped enclosure, they are unacceptable in a sealed,
evacuated unit because of outgassing of volatile components.
Moreover, the fabrication of an electron device often involves high
temperature joining operations that would destroy an organic
polymer. An inorganic coating having negligible vapour pressure is
thus highly desirable, but is more difficult to form as a thin
layer from a printable composition. Thin films of insulating metal
oxide can be readily deposited in vacuo by evaporation or
sputtering, but for easy and economical processing a liquid
precursor is required which, together with desired conductive
particles, can be incorporated into an ink which can be
printed.
[0156] One type of liquid precursor is a liquid or soluble compound
that will decompose to form a metal oxide on heating. There are
many metal salts which will undergo such decomposition but which
form particulate powdery deposits rather than the required film. A
few, such as magnesium acetate, will form transparent coatings
under certain conditions, such as spraying onto hot glass, but
these tend to re-crystallise and to show poor adhesion.
Organo-metallic complexes can give better results but high
volatility leads to difficulty in confining the coating to the
required area, and processing is often made difficult by, for
example, their being extremely inflammable or even pyrophoric. One
range of practicable materials is to be found in sol-gels, which
can be produced from a wide range of elements. These materials will
readily form films by coalescence and drying from the liquid state,
and are generally compatible with a wide range of other
materials.
[0157] Control of the chemical nature of the insulator is essential
as this will determine its electrical properties, which in turn are
crucial to the field emission process. Amorphous silica has been
found to be one particularly suitable insulator and films can be
formed via organic or inorganic chemical routes. Other insulators
that may be used to good effect are amorphous alumina and
Laponite.
[0158] In the case of the organic-based approach, materials such as
silicones (polysiloxanes) maybe used. Equally, Arkles (U.S. Pat.
No. 5,853,808) describes the use of silsequioxane polymers as
precursors for the preparation of silica films. We have found these
materials to be useful alternatives to sol-gel dispersions in the
formulation of emitter inns. These materials are reversably soluble
in a number of solvents, for example methoxypropanol. One polymer,
.beta.-chloroethylsilsesquioxane, has been found to be particularly
useful. It is known that .beta.-chloroethylsilsesquioxane and other
silsesquioxanes such as hydrogensilsequioxane and
acetoxysilsesquioxane yield ormosils (organically modified silicas)
on heating or exposure to ultraviolet irradiation in the presence
of ozone. Since, for example, some modified polysiloxanes are water
soluble, the organic-based approach does not necessarily imply
organic solvents.
[0159] In the case of the inorganic approach, sol-gel materials
offer wide opportunities for easy variation of composition and are
compatible with solvent mixtures such as: water; alcohol and water;
and alcohol, acetone and water.
[0160] As previously pointed out, CHTR printing inks to deposit
structures for field emission often have two unusual features that
make their formulation particularly challenging.
[0161] 1. The vehicle component of the ink is fugitive, being
decomposed and/or volatilised by subsequent drying and heat
treatment to leave the insulator or insulator precursor, and
comprises a much greater proportion of the ink than is normal in
other screen-printing arts--e.g. inks for decoration of ceramics or
thick film hybrid circuits.
[0162] 2. The proportion of solid particles in the ink is extremely
low by conventional screen printing ink standards.
[0163] The first of these features restricts the choice of
materials that can be incorporated to control the rheological
properties of the ink. Any fugitive polymer introduced to increase
the viscosity has to decompose and volatilise at temperatures that
will not damage the rest of the structure e.g. deformation of the
glass substrate. In practice this is likely to entail removal at a
temperature that is not greater than 450.degree. C. To ease this
process it is also desirable to use the minimum amount of any
additive. For inks based on organic solvents, exemplary materials
are ethyl cellulose, usually dissolved in terpineol, and
methacrylate polymers dissolved in a variety of mixtures of ester
and hydrocarbon solvents.
[0164] The insulator may then be introduced via suitable precursor.
In the case of silica it can be introduced by means of, for
example, a suitable substituted siloxane (silicone),
silsesquioxanes or silica sol-gel. Clean and complete thermal
decomposition of these polymers is achieved by around 350.degree.
C. to yield silica or an ormosil.
[0165] Inks based on water not only avoid problems associated with
the use of inflammable and harmful solvents, they allow the use of
a wide range of water-based sol-gel materials for the formation of
the insulator component of the emitter structure. The increase in
viscosity required for printing can be achieved by the use of water
soluble polymers such as poly(vinyl alcohol) or hydroxypropyl
cellulose (HPC)--both are readily removed by thermal
volatilisation. Poly(vinyl alcohol) or HPC have further advantages
when used with sol-gel materials in that each can itself become
incorporated (reacted) with the sol by condensation of the hydroxyl
groups of the gel with those of the polymer side chains. This leads
to a beneficial rise in viscosity of the ink allowing the use of
reduced concentrations of polymer.
[0166] The control of rheology is also affected by the generally
low particle loading required in these inks. Whereas in most
printing inks, the particle concentration is large enough to make a
major contribution to the viscosity of the ink, in some versions of
these inks any effect of the particles on rheological properties is
negligible and the rheological properties of the ink are, in the
main, those of the vehicle and precursor or vehicle, precursor and
filler. This is of particular importance with inks for
screen-printing where a high particle loading helps to prevent the
ink from bubbling as it is passed through the fine mesh of the
printing screen. In the absence of this effect, these inks need an
alternative mechanism to prevent bubbling during printing. One
means is to incorporate an anti-foaming and/or air release agent
into the ink. Polymer and ink additive manufacturers offer a
variety of materials for this purpose such as the longer chain
aliphatic alcohols or proprietary mineral oil type defoamants.
Butyl cellosolve and n-octanol have been found to be effective with
poly(vinyl alcohol), and n-Octanol is effective with hydroxypropyl
cellulose. When used with sol-gels, the condensation of poly(vinyl
alcohol) or hydroxypropyl cellulose side chains with the polymer
induces a slight gelation of the solution which is highly
advantageous as it increases the viscosity for a given amount of
polymer. The gel also helps to eliminate any bubbling during screen
printing.
[0167] Some polymers may also act as dispersants by both preventing
the particles from flowing in the ink and by coating the particles
leading to steric repulsion.
[0168] The ink may optionally contain: dispersing agents; a
preservative; a retarder (to slow down the rate of drying of the
ink); a wetting agent to improve wetting of the ink on the
substrate.
[0169] The material for printing is usually, but not necessarily, a
single liquid phase. However, the particulate component maybe
dispersed using suitable surfactants in, for example, a mineral oil
phase which is immiscible with the polymer and majority of solvents
used.
[0170] Our prior patent publications (e.g. GB 2 304 989, GB 2 332
089) teach that the threshold field for electron emission is
controlled by factors that include the enhancement of the
macroscopic electric field by the particle--the so-called .beta.
factor. Moving now to FIG. 4, graphite, one of our preferred
particles, generally has a flake-like habit and, as a result the
particles 400 tend to be pulled down onto the surface of the
substrate 401 by the liquid phase of the ink (not shown). It will
be clear to those skilled in the art that in this state its .beta.
factor is at its lowest value. FIG. 5 shows the structure of a film
as the printed thickness is increased: as before, the insulator
phase is not shown. In this case the flake-like particles now form
a more chaotic structure with many 410 tilted upwards increasing
their associated .beta. factors. The .beta. factor maybe by
increased further by using selected grades of graphite which as a
result of the specific milling conditions used, have a high
proportion of acicular particles. Both arrangements have two
potential shortcomings. Firstly there are many voids 411 and, if
the insulator concentration in the ink is increased to fill them,
the resulting insulator layer over the particles may be too thick
for low field emission. Secondly, with the correct amount of
insulator for emission, the film maybe both mechanically weak and
porous, making it difficult to build gate and other structures on
top of it. FIG. 6 shows a method by which this problem maybe
overcome. More equiaxed particles such as carbon blacks 420, of
sizes chosen to fill the voids, are added to the flake-like
particles 421. Carbon blacks are in many ways ideal since the small
primary particles aggregate to form structures not unlike bunches
of grapes and these aggregates then go on to form larger
agglomerates. Not only do the equiaxed particles increase the
strength and density of the film, they also have a tendency to prop
up the flakes and, consequently, increase their associated .beta.
factors. Another approach is to use graphite which has been milled
to increase the proportion of equiaxed particles which will
protrude above the surface irrespective of their orientation
relative to it and also help to prop up any less equiaxed
particles.
[0171] Our prior patent GB 2 304 989 describes the use of resistive
ballast layers between the emitting particles and the conducting
substrate. FIG. 7a shows such an arrangement formed from a thick
film as in FIG. 5, with substrate 401, conductive particles 430 and
insulator 431. Following the usual electro-forming stage,
conducting channels 432 and 433 are established between the
conductive particles 430. The channels 433 at the surface become
the sources of electron emission and the channels 432 within the
body of the layer help to stabilise the emitted current. Thus
region 440 of the film is the emitter layer as taught in our
previous work and region 441 provides a ballast layer.
[0172] FIG. 7b shows how the above concept maybe extended to
increase the resistive ballasting effect. In this case resistive
particles such as silicon carbide 450 are mixed with smaller
conductive particles 451 (e.g. graphite) known to give the best
emission in conjunction with the insulator layer 452. Relative
sizes and concentrations are chosen such that the smaller
conducting particles do not collectively form conducting pathways
through the resistive layer. The smaller conductive particles form
MIMIV emitter sites 453 on the surface of the larger resistive
particles. Field enhancement at the emitting sites is enhanced
above their values on a flat substrate by the .beta. factors of the
larger resistive particles augmenting those of the smaller
conductive particles that decorate their surfaces. Electrical
connection between the resistive particles 450 is by percolation
through the matrix afforded by the insulating material 452. Thus,
region 461 of the film is the emitter layer as taught in our
previous work and region 460 provides a ballast layer. Of course
the larger particles need not be resistive if only an increase in
.beta. factor is required. In such an arrangement, an ink with two
sizes of, for example, graphite particles may be formulated to
reduce the operating field of the finished emitter. The properties
of the smaller particles may also be carefully chosen for good
emission e.g. good crystallinity and/or acicular shape.
[0173] Preferred embodiments of the present invention employ
graphite particles at least partly coated or decorated with
amorphous silica which is doped and/or heavily defective. By
"heavily defective" is meant silica in which the band edges are
diffuse with a plurality of states that may, or may not, be
localised such that they extend into the band-gap to facilitate the
transport of carriers by hopping mechanisms. By "doped" we mean
doping as it is described in our patent GB 2 353 631. However,
perfectly functional emitters maybe made using other insulator
systems e.g: alumina and Laponite.
[0174] Examples of CHTR ink formulations using the teachings of
this document are described below.
[0175] To avoid repetition a number of key materials are defined
below--all values given are typical and not absolute.
[0176] Graphite A is a high purity synthetic lamelliform material
with a d90 value of 6.5 micrometres measured using a Malvern
instrument. Its specific surface area measured using the BET method
is 20 square metres per gram. The Brunauer, Emmett, and Teller
(BET) method is described by the authors in Journal of American
Chem. Society. 60, 309, 1938. Its dibutylphthalate absorbtion is
164 grams per 100 grams.
[0177] Graphite B is a natural lamelliform material of with a
d.sub.90 value of 6.6 micrometres measured using a Malvern
instrument.
[0178] Graphite C is a high-purity synthetic lamelliform material
with a d.sub.90 value of 4.7 micrometres measured using a Malvern
instrument. Its specific surface area measured using the BET method
is 26 square metres per gram.
[0179] Graphite D1 is similar to Graphite A but the feedstock and
milling conditions chosen to enhance the proportion of equiaxed
particles. It has a d.sub.90 of 6.1 micrometres measured using a
Malvern instrument.
[0180] Graphite D2 is similar to Graphite A but the feedstock and
milling conditions chosen to enhance the proportion of acicular
particles. It has a d.sub.90 of 6.5 micrometres measured using a
Malvern instrument. Its specific surface area measured using the
BET method is 17 square metres per gram.
[0181] Carbon Nanotubes D3 are single and/or multi-walled carbon
nanotubes grown using the conventional arc-discharge method in a
helium atmosphere which are subsequently ground, acid washed, and
rinsed in de-ionised water.
[0182] Graphite E is a ball milled synthetic graphite of mixed
equiaxed and lamelliform particles with a size range up to 8
micrometres. Its specific surface area measured using the BET
method is 127 square metres per gram.
[0183] Graphite F is a natural material (Ceylon) with particle
sizes in the range 1 to 13 micrometres with a typical value of 6
micrometres measured using a Malvern instrument. Its specific
surface area measured using the BET method is in the range 9 to 21
square metres per gram.
[0184] Graphite G is a natural lamelliform material from with
particle sizes in the range 4 to 7 micrometres with a typical value
of 6 micrometres measured using a Malvern instrument. Its specific
surface area measured using the BET method is 11.6 square metres
per gram.
[0185] Silicon carbide H has particles with a d.sub.90 of 1.48
micron determined using a Malvern instrument. The free Silicon
content is less than 0.1% with 95% being beta-SiC. Its specific
surface area measured using the BET method is 11.78 square metres
per gram.
[0186] Graphite dispersion I is an aqueous paste-like dispersion of
Graphite A. It has a pH value of 5.5.+-.1.
[0187] Graphite dispersion J is an aqueous bead-milled
pre-dispersed colloidal graphite with 11% solids content. Of the
particulate phase 90% are submicron with less than 5% over 5
micrometres. Its pH value is greater than 10.
[0188] Graphite dispersion K is a stable colloidal graphite
suspension in mineral oil with a solids content of 20%. Of the
particulate phase 95% are finer than 1 micrometres.
[0189] Hydroxypropyl cellulose L has an average molecular weight of
140 000 determined by size exclusion chromatography.
[0190] Hydroxypropyl cellulose M has an average molecular weight of
370 000 determined by size exclusion chromatography.
[0191] Poly(vinyl alcohol) N is 88% partially hydrolysed polyvinyl
alcohol in 4% by volume aqueous solution at 20 degrees C. The
viscosity is 40 mPa.s
[0192] Silicon dioxide precursor P is a solution of
.beta.-chloroethylsilsesquioxane in methoxypropanol.
EXAMPLE 1
[0193]
1 Quantity by Materials weight Graphite A 1.50 15 wt % solution of
poly(vinyl 66.65 alcohol) N in de-ionised water De-ionised water
10.85 Silica sol-gel in iso-propanol 20.00 n-butanol 1.00
[0194] The graphite powder is first mixed with the poly(vinyl
alcohol) solution by means appropriate to the lot size. The sol-gel
is then added to the mixture and carefully incorporated. The large
difference in viscosity between the polymer solution and the
sol-gel may make mixing difficult. The sol-gel should be added a
little at a time, the falling viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring.
[0195] The ink is then placed in a well-sealed container and kept
at 60.degree. C. for 2 hours, then allowed to cool and to stand for
24 hours. This last step is essential to allow the stabilisation of
reaction between the sol-gel and the poly(vinyl alcohol) which
induces a slight gelation of the ink. This gelation modifies the
rheological properties of the ink and enables it to be
screen-printed.
[0196] The silica sol-gel was prepared from the following
materials:
2 Quantity by Materials weight Tetraethyl orthosilicate 20.83
Iso-propanol 48.50 4 vol % Nitric Acid 5.57
[0197] Measure the reactants into suitable containers, cover and
cool to approximately 5.degree. C.
[0198] In a cooled vessel mix the tetraethyl orthosilicate and
iso-propanol and stir to maintain a steady but vigorous agitation
of the mixture. Add the nitric acid, which catalyses the reaction
and seal. Stir for 2 hours, maintaining the temperature below
10.degree. C. Transfer the mixture to a storage vessel and store in
a refrigerator.
EXAMPLE 2
[0199]
3 Quantity by Materials weight Graphite A 7.50 10 wt % solution of
hydroxypropyl 71.5 cellulose L in water Aqueous silica sol-gel
20.00 1-octanol 1.00
[0200] The graphite powder is first mixed with the hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the falling viscosity of the mixture making
subsequent additions easier. Finally the 1-octanol is added and
mixed. The ink is then placed in a well-sealed container and kept
at 22.degree. C. and left to stand for 24 hours.
[0201] The aqueous silica sol-gel was prepared as follows:
4 Quantity by Materials weight Tetraethyl orthosilicate 27.8
De-ionized water 72.0 Concentrated nitric acid 0.2
[0202] The tetraethyl orthosilicate was added to the water at room
temperature and stirred vigorously and then the nitric acid was
added. The stirred mixture was then held at .sup..about.48.degree.
C. for one hour, at the end of which the mixture had become a clear
colourless liquid. This liquid was then transferred to a bottle and
refrigerated.
EXAMPLE 3
[0203] In an alternative preparation, the graphite can be
incorporated in the form of dispersions rather than dry particles
and the range of particle sizes increased by the use of mixed
dispersions:
5 Quantity by Materials weight Predispersed colloidal 6.8 graphite
J(filtered through 8 micrometer filter) Predispersed graphite
dispersion I 6.8 (filtered through 8 micrometer filter) Acetic acid
1 Hydroxypropyl cellulose solution 41 Silica sol-gel in
1,2-propanediol 4.5 De-ionised water 9.69 Butoxyethanol 17.1
[0204] The carefully stirred and filtered graphite dispersions are
mixed together and then acetic acid is added to adjust pH to
approximately 3. The hydroxypropyl cellulose solution and the
silica sol-gel were then added. The flow properties and viscosity
are adjusted with the additional butoxyethanol and water and the
composition roller milled to obtain a well dispersed, smooth
material for screen printing. The ink is then placed in a
well-sealed container and kept at 22.degree. C. and left to stand
for 24 hours.
[0205] The silica sol-gel was prepared from the following
materials:
6 Quantity by Materials weight Tetraethyl orthosilicate 28.05
1,2-propanediol 20.72 4 wt % Nitric Acid 6.00
[0206] Measure the reactants into suitable containers, cover and
cool to approximately 5.degree. C.
[0207] In a cooled vessel mix the tetraethyl orthosilicate and
1,2-propanediol and stir to maintain a steady but vigorous
agitation of the mixture. Add the nitric acid, which catalyses the
reaction and seal. Stir for 2 hours, maintaining the temperature
below 10.degree. C. Transfer the mixture to a storage vessel and
store in a refrigerator.
[0208] The hydroxypropyl cellulose solution was prepared as
follows:
7 Materials Quantity Hydroxypropyl cellulose L 30 g Ethanol 54 ml
1,2-propanediol 180 ml De-ionised water 126 ml
[0209] The solvents are placed in a stirred reaction flask fitted
with a heater and condenser. The solvent mixture is stirred
vigorously at room temperature and the polymer added slowly to
ensure that the powder is dispersed into the liquid. The flask is
then heated with continuous stirring to 80.degree. C., stirred at
this temperature for 15 minutes and then cooled to room
temperature. The solution should be clear and of uniform
viscosity.
EXAMPLE 4
[0210]
8 Quantity by Materials weight Predispersed colloidal graphite J 20
(filtered through 8 micrometer filter) Predispersed graphite
dispersion I 7.6 (filtered through 8 micrometer filter) Acetic acid
1.0 Hydroxypropyl cellulose solution 31 Silica sol-gel in
iso-propanol 7.37 Butoxyethanol 3.0
[0211] The filtered graphite dispersions are mixed together and
then acetic acid is added to adjust pH to approximately 3. The
hydroxypropyl cellulose solution and the silica sol-gel were then
added. The flow properties and viscosity are adjusted with the
additional butoxyethanol and water and the composition roller
milled to obtain a well dispersed, smooth material for screen
printing. The ink is then placed in a well-sealed container and
kept at 22.degree. C. and left to stand for 24 hours.
[0212] The silica sol-gel was prepared in the same manner as
described in Example 1. The hydroxypropyl cellulose solution was
prepared in the same manner as described in Example 3.
EXAMPLE 5
[0213]
9 Quantity by Materials weight Graphite A 1.3 Silica sol-gel in
1,2-propanediol 4.55 1,2-propandiol 24.37 Hydroxypropyl cellulose
solution 32 Iso-propanol 11.73 De-ionised water 9.5 Butoxyethanol
17.36
[0214] The graphite and silica sol gel are mixed and the
propane-1,2-diol added. The ink is mixed to a smooth paste using
ultrasonic agitation. The polymer and remaining solvents are then
added before triple roll milling the ink several times to ensure
uniformity. The ink is then placed in a well-sealed container and
kept at 22.degree. C. and left to stand for 24 hours.
[0215] The silica sol-gel was prepared in the same manner as
described in Example 3. The hydroxypropyl cellulose solution was
prepared in the same manner as described in Example 3.
EXAMPLE 6
[0216]
10 Quantity by Materials weight Graphite A 2.39 5 wt % Laponite
solution in de- 7.5 ionised water 1,2-Propanediol 15 Hydroxypropyl
cellulose solution 19 Butoxyethanol 12
[0217] The graphite and Laponite solutions are mixed with the
1,2-propanediol with the aid of ultrasonic agitation. The
hydroxypropyl cellulose solution and butoxy ethanol are stirred in
and the material passed several time through a triple roll mill to
obtain uniform consistency. The ink is then placed in a well-sealed
container and kept at 22.degree. C. and left to stand for 24
hours.
[0218] The hydroxypropyl cellulose solution was prepared in the
same manner as described in example 3. However, in this case, 22.5
g of hydroxypropyl cellulose was used.
[0219] Laponite is a commercial, synthetic, clay mineral supplied
by.
[0220] Laporte Industries Ltd.
[0221] Moorfield Road
[0222] Widnes
[0223] Cheshire WA8 0JU
[0224] United Kingdom
EXAMPLE 7
[0225]
11 Quantity by Materials weight Graphite G 3.00 15 wt % solution of
poly(vinyl 10.00 alcohol) N in de-ionised water Silica sol-gel in
iso-propanol 5.00 1,2-propanediol 2.00 1-Octanol 0.20
[0226] The graphite powder is first mixed with the poly(vinyl
alcohol) solution by means appropriate to the lot size. The sol-gel
is then added to the mixture and carefully incorporated. The large
difference in viscosity between the polymer solution and the
sol-gel may make mixing difficult. The sol-gel should be added a
little at a time, the falling viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring. The ink is then
placed in a well-sealed container and kept at 22.degree. C. and
left to stand for 24 hours.
[0227] The poly(vinyl alcohol) solution and the silica sol-gel were
both prepared in the same manner as described in Example 1.
EXAMPLE 8
[0228]
12 Quantity by Materials weight Graphite F 3.00 15 wt % solution of
poly(vinyl 10.00 alcohol) N in de-ionised water Silica sol-gel in
iso-propanol 5.00 1,2-propanediol 2.00 1-Octanol 0.20
[0229] The graphite powder is first mixed with the poly(viny
alcohol) solution by means appropriate to the lot size. The sol-gel
is then added to the mixture and carefully incorporated. The large
difference in viscosity between the polymer solution and the
sol-gel may mixing difficult. The sol-gel should be added a little
at a time, the failing viscosity of the mixture making subsequent
additions easier. The final additions of water and solvent require
no more than thorough stirring. The ink is then placed in a
well-sealed container and kept at 22.degree. C. and left to stand
for 24 hours.
[0230] The poly(viny alcohol) solution and the silica sol-gel
solution were both prepared in the same manner as described in
Example 1.
EXAMPLE 9
[0231]
13 Quantity by Materials weight Graphite E 0.9 Hydroxypropyl
cellulose solution 19.23 Silica sol-gel in iso-propanol 7.50
De-ionised water 4.93 1,2-propanediol 13.13 Iso-propanol 4.32
[0232] The graphite powder is first mixed with the Hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the falling viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring. The ink is then
placed in a well-sealed container and kept at 22.degree. C. and
left to stand for 24 hours.
[0233] The hydroxypropyl cellulose solution was prepared in the
same manner as described in Example 3. The silica sol-gel solution
was prepared in the same manner as described in Example 1.
EXAMPLE 10
[0234]
14 Quantity by Materials weight Graphite B 0.75 Hydroxypropyl
cellulose solution 17.95 Silica sol-gel in iso-propanol 5.00
De-ionised water 5.50 1,2-propanediol 14.04 Iso-propanol 6.76 Butyl
cellosolve 10.00
[0235] The graphite powder is first mixed with the Hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the failing viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring. The ink is then
placed in a well-sealed container and kept at 22.degree. C. and
left to stand for 24 hours.
[0236] The hydroxypropyl cellulose solution was prepared in the
same manner as described in Example 3. The silica sol-gel solution
was prepared in the same manner as described in Example 1.
EXAMPLE 11
[0237]
15 Quantity by Materials weight Graphite D1 or Graphite D2 0.75
Hydroxypropyl cellulose solution 17.95 Silica sol-gel in
iso-propanol 5.00 De-ionised water 5.50 1,2-propanediol 14.04
Iso-propanol 6.76 Butyl cellosolve 10.00
[0238] The graphite powder is first mixed with the Hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the falling viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring. The ink is then
placed in a well-sealed container and kept at 22.degree. C. and
left to stand for 24 hours.
[0239] The hydroxypropyl cellulose solution was prepared in the
same manner as described in Example 3. The silica sol-gel solution
was prepared in the same manner as described in Example 1.
EXAMPLE 12
[0240] An example of one suitable formulation is as follows
16 Quantity by Materials weight Graphite A 1.50 4 wt % solution of
hydroxypropyl 88.50 cellulose M in 1-methoxy-2- propanol Silicon
dioxide precursor P 10.00
[0241] The powder is first mixed with the hydroxypropyl cellulose
solution by means appropriate to the lot size. The silicon dioxide
precursor P is then added to the mixture and carefully
incorporated. The large difference in viscosity between the polymer
solution and the precursor P may make mixing difficult. The
precursor should be added a little at a time, the failing viscosity
of the mixture making subsequent additions easier. The final
additions of precursor require no more than thorough stirring. The
ink is then placed in a well-sealed container then allowed to stand
for 24 hours.
EXAMPLE 13
[0242]
17 Quantity by Materials weight Graphite C 1.00
Hydroxypropylcellulose solution 43.67 in 1,2-propanediol Silica
sol-gel in 1,2-propanediol 5.33
[0243] The graphite powder is first mixed with the Hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the falling viscosity of the mixture making
subsequent additions easier. The final additions of water and
solvent require no more than thorough stirring. The ink is then
placed in a well-sealed container and kept at 22.degree. C. and
left to stand for 24 hours.
[0244] The hydroxypropyl cellulose solution was prepared in the
same manner as described in Example 3 while heating from
temperature. However, in this case, it contains:
18 Quantity by Materials weight Hydroxypropyl cellulose L 36.0
1,2-propanediol 364.0
[0245] The silica sol-gel was prepared from the following
materials:
19 Quantity by Materials volume Tetraethyl orthosilicate 74
1,2-propanediol 108 De-ionised water acidified to pH 1 18 using
nitric acid
[0246] In a vessel mix the tetraethyl orthosilicate and
1,2-propanediol and stir to maintain a steady but vigorous
agitation of the mixture. Add the acidifed water, which catalyses
the reaction and seal. Stir for 2.5 hours, maintaining the
temperature at 20.degree. C. Transfer the mixture to a storage
vessel and store in a refrigerator.
EXAMPLE 14
[0247] The material for printing is usually, but not necessarily, a
single liquid phase. In the following example the graphite is
supplied in a mineral oil phase which is immiscible with the
polymer solution and majority of solvents used. However the
graphite rich phase can be stabilised by suitable surfactants:
20 Quantity by Material weight Graphite dispersion in mineral oil K
2.5 polyethylene glycol mono(4- 1 (1,1,3,3-
tetramethylbutyl)phenyl)ether Hydroxypropyl cellulose solution 42.8
Silica sol-gel in iso-propanol 3 1,2-propandiol 5 Xylene 1.5
Methoxypropanol 1.5 Octanol 1
[0248] The graphite in mineral oil is mixed with polyethylene
glycol mono(4-(1,1,3,3-tetramethylbutyl)phenyl)ether surfactant and
the remaining components. The graphite is in the minor, mineral oil
phase, and after printing is distributed in sharply localised
areas.
[0249] The hydroxypropyl cellulose solution was prepared in the
same manner as described in example 3. However, in this case, 22.5
g of hydroxypropyl cellulose was used. The silica sol-gel was
prepared in the same manner as described in Example 1.
EXAMPLE 15
[0250]
21 Quantity by Material weight Graphite A 1.5 Silicone oil of 10
cps viscosity 0.1 Hydroxypropyl cellulose solution 29 Butoxyethanol
15 1,2-propanediol 10 Silica sol-gel in iso-propanol 2
[0251] The graphite, silicone oil and polymer are mixed to a smooth
paste. The solvents are then added and finally the silica sol-gel
is mixed in before triple roll milling. The ink is then placed in a
well-sealed container and kept at 22.degree. C. and left to stand
for 24 hours. The hydroxypropyl cellulose solution was prepared in
the same manner as described in Example 3. The silica sol-gel was
prepared in the same manner as described in Example 1.
EXAMPLE 16
[0252]
22 Quantity by Materials weight Graphite A 7.50 10 wt % solution of
hydroxypropyl 71.5 cellulose L in water Aqueous alumina sol-gel
20.00 1-octanol 1.00
[0253] The graphite powder is first mixed with the hydroxypropyl
cellulose solution by means appropriate to the lot size. The
sol-gel is then added to the mixture and carefully incorporated.
The large difference in viscosity between the polymer solution and
the sol-gel may make mixing difficult. The sol-gel should be added
a little at a time, the failing viscosity of the mixture making
subsequent additions easier. Finally the 1-octanol is added and
mixed. The ink is then placed in a well-sealed container and kept
at 22.degree. C. and left to stand for 24 hours.
[0254] The aqueous alumina sol-gel was prepared as follows:
23 Materials Quantity Aluminium tri-sec-butoxide 17.2 g De-ionized
water 126 ml Concentrated nitric acid 0.31 ml
[0255] The aluminium tri-sec-butoxide was hydrolysed in the water
at 75.degree. C. with vigorous stirring for 20 minutes. The
solution was then heated to 85.degree. C. and the nitric acid
added. The mixture was then stirred continuously at this elevated
temperature for about 20 hours, at the end of which the mixture had
become a clear colourless liquid. This liquid was then transferred
to a bottle and refrigerated.
EXAMPLE 17
[0256]
24 Quantity by Materials weight Predispersed colloidal graphite J
14.7 (filtered through 8 micrometer filter) Silicon carbide H 0.45
Hydroxypropyl cellulose solution 30 Silica sol-gel in iso-propanol
5.5 1,2-propanediol 4.8 Butoxyethanol 6.4
[0257] The filtered graphite dispersion, silicon carbide and silica
sol gel are mixed, acidified and gently heated at 65-70.degree. C.
until coagulation occurs. The hydroxypropyl cellulose and solvents
are then added and the mixture triple roll milled until of uniform
consistency. The ink is then placed in a well-sealed container and
kept at 22.degree. C. and left to stand for 24 hours.
[0258] The silica sol-gel was prepared in the same manner as
described in Example 1. The hydroxypropyl cellulose solution was
prepared in the same manner as described in Example 3.
EXAMPLE 18
[0259]
25 Quantity by Materials weight Graphite A 2.27 2 wt % Laponite
solution in de- 18.75 ionised water Acetic acid 1 14 wt % silica
solgel in propan2ol 1.25 Hydroxypropyl cellulose solution 24
[0260] The graphite particles and Laponite solution are mixed,
acidified and heated to 100.degree. C. for 5 minutes. The silica
sol-gel and hydroxypropyl cellulose are then added and the mixture
passed through a triple roll mill. The ink is then placed in a
well-sealed container and kept at 22.degree. C. and left to stand
for 24 hours.
[0261] The hydroxypropyl cellulose solution and the silica sol-gel
were both prepared in the same manner as described in example 3.
However, in this case, 22.5 g of hydroxypropyl cellulose was used
for the polymer solution. Laponite may be obtained from the same
Laporte Industries address given in Example 6.
EXAMPLE 19
[0262]
26 Quantity by Materials weight Carbon Nanotubes D3 1.1 Silica
sol-gel in 1,2-propanediol 4.55 1,2-propandiol 24.37 Hydroxypropyl
cellulose solution 32 Iso-propanol 11.73 De-ionised water 9.5
Butoxyethanol 17.36
[0263] The carbon nanotubes and silica sol gel are mixed and the
propane-1,2-diol added. The ink is mixed to a smooth paste using
ultrasonic agitation. The polymer and remaining solvents are then
added before triple roll milling the ink several times to ensure
uniformity. The ink is then placed in a well-sealed container and
kept at 22.degree. C. and left to stand for 24 hours.
[0264] The silica sol-gel was prepared in the same manner as
described in Example 3. The hydroxypropyl cellulose solution was
prepared in the same manner as described in Example 3.
[0265] Nanotubes of materials other than carbon may be used in
alternative formulations.
[0266] Inks may also be prepared using combinations of the
following functional materials.
[0267] Thickening agents: Ethyl cellulose, hydroxyethyl cellulose,
carboxymethyl cellulose, methylhydroxypropyl cellulose,
hydroxypropyl cellulose, xanthan gum, guar gum.
[0268] Anti-foaming agents: Emulsions of organic polymers and
organic metal-compounds for aqueous based inks (e.g. EFKA-2526,
EFKA-2527); silicone free defoaming substances in alkylbenezene
(e.g. EFKA-2720).
[0269] Levelling agents: Fluorocarbon modified polyacrylate in sec.
butanol for both aqueous and non-aqueous inks (e.g. EFKA-3772);
organically modified polysiloxane in isobutanol (e.g. EFKA-3030);
solvent-free modified polysiloxane (e.g. EFKA-3580).
[0270] Wetting agents: Unsaturated polyamide and acid ester salt in
xylene, n-butanol and monopropyleneglycol (e.g. EFKA-5044); anionic
wetting agents of alkylol ammonium salts of a high molecular weight
carboxylic acid in water (e.g. EFKA-5071).
[0271] Preservatives: phenol, formaldehyde.
[0272] Air-release agents: silica particles, silicones.
[0273] Retarder: 1,2-propanediol, terpineol.
[0274] Dispersing agents: Modified polyurethane in butylacetate,
methoxypropylacetate and sec. butanol (e.g. EFKA-4009); modified
polyacrylate in methoxypropanol (EFKA-4530), polyethylene glycol
mono(4-(1,1,3,3-tetramethylbutyl)phenyl)ether.
[0275] Methylhydroxypropyl cellulose and other thickeners at lower
concentrations may also serve this function. In fact many additives
may have multiple functionality.
[0276] EFKA products may be obtained from:
[0277] EFKA Additives bv
[0278] Innovatielaan 11
[0279] 8466 SN Nijehaske
[0280] The Netherlands
[0281] Since the printing properties are not controlled by low
concentration of particles they can be removed entirely to leave a
CHTR ink for the printing of, for example, the gate insulator of a
field emission device.
[0282] The above CHTR inks all have theological properties suitable
for screen printing. Their typical rheological properties are
illustrated by the exemplary flow curve shown in FIG. 13b. The
theological measurements were made using a Bohlin CV 120 rheometer
using cone and plate geometry. FIG. 13b shows the clearly different
rheological properties of a conventional proprietary high
resolution thick film printing paste measured on the same
instrument.
[0283] For displays work cathode tracks are typically screen
printed onto suitable conducting films such as gold upon a glass
substrate. Said films may be deposited by vacuum evaporation,
sputtering or directly screen printed using so the called resinate
or bright gold inks--see the Applicant's patent GB 2 330 687. Said
printing is generally performed using a 400 mesh stainless steel
screen with an approximately 13 micrometre thick emulsion
layer.
[0284] After the substrates have been printed they are transferred
to hotplates under the following conditions: a) 10 minutes at
50.degree. C.-measured surface temperature of hotplate; b) 10
minutes at 120.degree. C.-measured surface temperature of hotplate.
The substrates are then transferred to an oven (air atmosphere)
according to the following profile: ambient to 450.degree. C. at
10.degree. C./min; isotherm at 450.degree. C. for 120 minutes;
followed by cooling naturally to room temperature.
[0285] Post-cure treatments such as gentle ultrasonic cleaning or
tacky rollers maybe used to remove any loose particles.
[0286] FIG. 14a, wherein the emitter patches are bright and
dimension 1400 is 500 micrometre, shows an example of simulated
pixel patches printed using the inks described herein FIG. 14b
wherein the emitter patches are bright and dimension 1401 is 300
micrometre and dimensions 1402 and 1403 are .sup..about.60
micrometre, shows an example of fine feature printing for, say, a
colour pixel triad.
[0287] The flatness of the finished film is an important parameter,
as it affects the ease by which subsequent structures, such as
gates, can be built upon the emitter layer. The best examples of
the inks described herein produce layers with an average roughness
of .sup..about.140 nm with a root mean square value of
.sup..about.70 nm when measured using a Burleigh Horizon
non-contact optical profilometer using a .times.10 Mirau
objective.
[0288] In FIGS. 15a and 15b, we see how printing and emitting
properties maybe adjusted by controlling the porosity of the
substrate. FIG. 15a shows a substrate 1501 with porous layer 1502
on top of which is a just-printed CHTR emitter ink layer 1503 with
conducting particles 1504. The applicant has found that the best
emission is obtained when the insulator film thickness over the
particles is a few tens of nanometres. During natural ding, surface
tension thins the insulator precursor layer over the convex upper
regions of the particles, leading to a desirable thinning process.
However, there is a race between this natural thinning process and
the drying of the film which, at a certain point, locks in the
thickness so reached. Moving now to FIG. 15b, we can seen how this
beneficial thinning process may be speeded up by the presence of
the porous layer 1502 below the printed ink layer that wicks away
some of the liquid component of the ink 1505 before it can dry. We
have found reductions in emission threshold field of
.sup..about.1.5 V/micrometre, using this approach.
[0289] The porous layer 1502 maybe beneficially formulated to have
resistive properties and so serve the additional function of a
ballast layer.
[0290] Using one of the previously mentioned examples to form the
basis of a screen-printed and heat-cured cold cathode layer, FIG.
9a, wherein dimension 900 is 11.2 mm, shows its performance
measured using an emission image. The cold cathode vas disposed as
a 1 cm.sup.2 circular disk on a gold coated borosilicate glass
slide, and mounted 0.25 mm away from a tin oxide coated glass anode
in a vacuum system. The voltage applied to the diode was varied
under computer control, with images of the electron bombardment
induced fluorescence on the tin oxide coated anode being viewed by
a CCD camera. The overall site density was limited by the 1 mA
current limit of the apparatus being used. Hence, the image
necessarily shows the sites with the lowest threshold field for
emission, giving an indication of the uniformity of sites of this
nature. For clarity of view and to facilitate reproduction, the
view in FIG. 9a is shown in reverse video--that is, original light
spots against a dark background are shown in the figure as dark
spots against a light background.
[0291] FIG. 9b shows a voltage-current characteristic for the same
sample as above, measured using the same equipment used to record
the image in FIG. 9a. It shows that a macroscopic field lower than
10 V/micrometre delivered more than 10 micro-amperes of
current.
[0292] FIG. 11 shows another emission image for another one of the
examples (again in reverse video). This had an extremely high site
density in one portion of the disk, the overall emitting area again
limited by the 1 mA equipment limit. Dimension 1101 is
.sup..about.3 mm and the site density determined using image
analysis software was .sup..about.27 000 cm.sup.-2.
[0293] FIG. 10a is a frequency histogram of threshold field for
forty-nine separately tested areas on a sample formulated using one
of the previous examples. The data was obtained by using a 350
micrometres diameter probe scanned 50 micrometres above the surface
of the sample in a computer-controlled vacuum test system. This
probe test provides a statistical distribution of the threshold
fields in an area defined by the 350 micrometres diameter probe.
FIG. 10b is a similar frequency histogram of the same sample
generated using a 35 micrometres diameter probe scanned 25
micrometres above the surface of the sample in the same test
system.
[0294] FIG. 12 shows current maps generated using the same probes
over two different areas of sample. In these images, light grey
pixels indicate 100 nA current and black squares indicate less than
1 nA current. In the first case 1200, a 5 mm by 10 mm scanned area,
measured using the 350 micrometres diameter probe, shows saturation
at a macroscopic field of 15 V/micrometre. In the second part 1201,
the 1 mm by 1 mm scanned area, measured using the 35 micrometres
diameter probe, shows emission site structure at 26 V/micrometre
commensurate with a site density of .sup..about.300 000 sites
cm.sup.-2.
[0295] The field electron emission current available from improved
emitter materials such as are disclosed above maybe used in a wide
range of devices including (amongst others): field electron
emission display panels; lamps; high power pulse devices such as
electron MASERS and gyrotrons; crossed-field microwave tubes such
as CFAs; linear beam tubes such as klystrons; flash x-ray tubes;
triggered spark gaps and related devices; broad area x-ray sources
for sterilisation; vacuum gauges; ion thrusters for space vehicles
and particle accelerators.
[0296] Examples of some of these devices are illustrated in FIGS.
8a, 8b and 8c.
[0297] FIG. 8a shows an addressable gated cathode as might be used
in a field emission display. The structure is formed of an
insulating substrate 500, cathode tracks 501, emitter layer 502,
focus grid layer 503 electrically connected to the cathode tracks,
gate insulator 504, and gate tracks 505. The gate tracks and gate
insulators are perforated with emitter cells 506. A negative bias
on a selected cathode track and an associated positive bias on a
gate track causes electrons 507 to be emitted towards an anode (not
shown).
[0298] The reader is directed to our patent GB 2 330 687 for
further details of constructing Field Effect Devices.
[0299] The electrode tracks in each layer may be merged to form a
controllable but non-addressable electron source that would find
application in numerous devices.
[0300] FIG. 8b shows how the addressable structure 510 described
above may joined with a glass fritt seal 513 to a transparent anode
plate 511 having upon it a phosphor screen 512. The space 514
between the plates is evacuated, to form a display.
[0301] Although a monochrome display has been described, for ease
of illustration and explanation, it will be readily understood by
those skilled in the art that a corresponding arrangement with a
three-part pixel may be used to produce a colour display.
[0302] FIG. 8c shows a flat lamp using one of the above-described
materials. Such a lamp may be used to provide backlighting for
liquid crystal displays, although this does not preclude other
uses, such as room lighting.
[0303] The lamp comprises a cathode plate 520 upon which is
deposited a conducting layer 521 and an emitting layer 522. Ballast
layers as mentioned above (and as described in our other patent
applications mentioned herein) may be used to improve the
uniformity of emission A transparent anode plate 523 has upon it a
conducting layer 524 and a phosphor layer 525. A ring of glass
fritt 526 seals and spaces the two plates. The interspace 527 is
evacuated.
[0304] The operation and construction of such devices, which are
only examples of many applications of embodiments of this
invention, will readily be apparent to those skilled in the art. An
important feature of preferred embodiments of the invention is the
ability to print an emitting pattern, thus enabling complex
multi-emitter patterns, such as those required for displays, to be
created at modest cost. Furthermore, the ability to print enables
low-cost substrate materials, such as glass to be used; whereas
micro-engineered structures are typically built on high-cost single
crystal substrates. In the context of this specification, printing
means a process that places or forms an emitting material in a
defined pattern. Examples of suitable processes to print these inks
are (amongst others): screen printing or offset lithography. If
patterning is not required techniques such as wire roll coating
(K-coaters) or blade coating may also be used.
[0305] Devices that embody the invention maybe made in all sizes,
large and small. This applies especially to displays, which may
range from a single pixel device to a multi-pixel device, from
miniature to macro-size displays.
[0306] In this specification, the verb "comprise" has its normal
dictionary meaning, to denote nonexclusive inclusion. That is, use
of the word "comprise" (or any of its derivatives) to include one
feature or more, does not exclude the possibility of also including
further features.
[0307] The reader's attention is directed to all papers and
documents which are filed concurrently with or previous to this
specification in connection with this application and which are
open to public inspection with this specification, and the contents
of all such papers and documents are incorporated herein by
reference.
[0308] All of the features disclosed in this specification
(including any accompanying claims, abstract and drawings), and/or
all of the steps of any method or process so disclosed, maybe
combined in any combination, except combinations where at least
some of such features and/or steps are mutually exclusive.
[0309] Each feature disclosed in this specification (including any
accompanying claims, abstract and drawings), maybe replaced by
alternative features serving the same, equivalent or similar
purpose, unless expressly stated otherwise. Thus, unless expressly
stated otherwise, each feature disclosed is one example only of a
generic series of equivalent or similar features.
[0310] The invention is not restricted to the details of the
foregoing embodiment(s). The invention extends to any novel one, or
any novel combination, of the features disclosed in this
specification (including any accompanying claims, abstract and
drawings), or to any novel one, or any novel combination, of the
steps of any method or process so disclosed.
* * * * *